This invention relates to an equipment for laser machining and a method for laser machining. Specifically, this invention relates to employable for laser a method and apparatus for laser ablation machining. More specifically, the invention relates to a method and apparatus for laser ablation machining using a dielectric mirror mask. Particularly, the invention relates to employable for laser ablation machining conducted for the purpose to produce a via hole to the insulator layer of a multi-layered circuit board and a method for laser ablation machining for the purpose of producing a via hole in the insulator layer of a multi-layered circuit board.
Infrared ray laser machining employing YAG lasers, or CO.sub.2 lasers or the like rays one of the infrared rays is widely known. Since laser machining employing infrared rays depends on thermal energy, such machining has a drawback in that it may cause thermal damage to objects; in the neighborhood of the machined area. Moreover, since infrared ray laser machining involves the use of a laser beam having a small cross-sectional area which is scanned along a surface to be machined, it is difficult to increase the machining rate. Furthermore, since it is difficult to confine infrared rays to; a beam having a small cross-sectional area, laser machining employing infrared rays is not suitable for micro machining of the sort necessary for producing via holes.
In order to achieve micro machining, a laser ablation machining procedure employing an excimer laser was developed. An excimer laser is a gas laser which emits strong ultraviolet rays depending on excitons of a rare gas and a halogen. The ordinary strength of an excimer laser is approximately 100 MW/cm.sup.2 at the peak of pulse. Since ordinary substances readily absorb ultraviolet rays, irradiation with ultraviolet rays generated by an excimer laser generally may result in the instant dissolution and vaporization of; the surface of the irradiated substance. Laser ablation machining employs this phenomenon. Excimer lasers such as KrF lasers are widely employed laser (248 nm), XeCl lasers (308 nm) and ArF lasers (193 nm).
Laser ablation machining has the following advantages:
1. Since laser ablation does not employ thermal energy but rather depends on dissolution of chemical bonds by the generated excimer laser ultraviolet rays, the finished shapes are arcuracte and beautiful;
2. Since laser ablation does not employ a laser beam but rather employs a broad laser flux which is assisted by a mask, accurate machining is readily achieved;
3. In particular, ArF lasers are suitable for machining polymers such as polyimide resins and the like.
Production of via holes in the insulator layer of multi-layered circuit boards is one of the promising uses for laser ablation machining.
A multi-layered circuit board which is available in the prior art is illustrated in FIG. 1. Referring to FIG. 1, insulator layers 22 and conductor layers 23 are piled alternately on a substrate 21, and the conductor layers 23 are connected with each other through via holes 24. Although a serial process has been developed for producing multi-layered circuit boards, such serial process has not yet been completely accepted, because of difficulty in forming via holes when such process is used. This is because certain types of polymers for example polyimide resin and the like, are preferably employed as the construction material for the insulator layers of such multi-layered circuit boards and these polymers are difficult to machine for the following reasons.
1. When such polymers are exposed to light, they do not necessarily allow light to penetrate all the way to the bottom of the layer, and as a result the accuracy of the via hole thus produced is unsatisfactory.
2. Such polymers do not allow for the production of via holes having an; aspect ratio greater than 3 when a wet etching process is employed.
3. It is difficult to obtain suitable photoresists for use in etching such polymers, and such etching processes are extremely complicated.
On the other hand, when excimer laser ablation machining is employed via holes having a high aspect ratio may be produced by a fairly simple process.
Excimer laser ablation machining is classified into two categories, one is known as the contact mask process, e.g. Siemens' "7500H90" process and the other is known as the projection process, e.g. IBM's "ES9000" process.
An example of the contact mask process will be described below with reference to the drawing.
Referring to FIG. 2, a polyimide resin layer 22a and a copper film 25 are laminated on a ceramic substrate 21a.
Referring to FIG. 3, a photoresist is coated on the copper film 25 to thus produce a photoresist layer 26. The photoresist layer 26 is selectively exposed to ultraviolet rays through a mask 27.
Referring to FIG. 4, the photoresist layer 26 is converted to an etching mask 26a by a developing process.
Referring to FIG. 5, the copper film 25 is converted to an etching mask 25a by an etching process employing the etching mask 26a of the photoresist layer 26 as a mask.
Referring to FIG. 6, a strong ultraviolet ray beam emitted by an excimer laser 11, e.g. a KrF laser, is irradiated into the polyimide resin layer 22a via a mirror 12, a lens 27 and the etching mask 25a of the copper film 25.
In this process, the intensity of each pulse of the ultraviolet ray beam irradiated onto the polyimide resin layer 22a must exceed 0.5 J/cm.sup.2. Therefore, the etching mask 25a is required to be a metal plate having a thickness larger than 0.2 mm. On the other hand, the required diameter of a via hole is 10 to 200 micrometers, in order for the produced via hole to have; a high aspect ratio. It is well known that via holes having high aspect ratios are difficult to produce. Moreover, the foregoing complicated lengthy process necessary for the contact mask process is a considerable drawback which can not be ignored.
An example of the projection process will next be described with reference to the drawings.
Referring to FIG. 7, a polyimide resin layer 22a is coated onto a ceramic substrate 21a, and an object to be machined is produced.
Referring to FIG. 8, an electric discharge machining process or photolithography is employed to convert a metal plate having a thickness of 0.1 to 0.3 mm into a metal mask 28 having an enlarged pattern consisting of openings 28a.
Referring to FIG. 9, the metal mask 28 is arranged in parallel to a lens 27a and to the object to be machined consisting of the ceramic substrate 21a with the polyimide resin layer 22a, and a mirror 12a which reflects a strong ultraviolet ray beam emitted by an excimer laser 11, e.g. a KrF laser, moves along a plane parallel to the metal mask 28. When the mirror 12a moves along the metal mask 28, the strong ultraviolet ray beam emitted by the excimer laser 11 is scanned along the surface of the polyimide resin layer 22a through the metal mask 28. In this process, the enlarged pattern of the metal mask is reduced by the lens 27a before being copied in the polyimide resin layer 22a by the excimer laser ablation phenomenon.
In this process, it is not easy to produce the metal mask 28 because such a metal plate is readily warped during the production process. Further, this process has a drawback because the maximum dimension of a frame in which one scanning pass of excimer laser ablation is allowed can not exceed 40 mm in diameter, thus requiring a large number of steps to be repeated for patterning the entire surface of the polyimide resin layer 22a.
The foregoing description clearly shows that both the contact mask process and the projection process are involved with drawbacks which need to be removed.
A thin mask which has a small absorption factor and a large reflection factor and is durable against irradiation by the strong ultraviolet ray beam emitted by an excimer laser, is assumed effective to improve the excimer laser ablation machining process.
In the meantime, it is known that a mask which is durable against a strong ultraviolet ray beam due to the large reflection factor is a dielectric mirror mask which is defined as a mask made of a dielectric mirror which is further defined as a piled plate in which two kinds of layers having refraction factor different from each other are alternately piled.